The present invention relates to a method and apparatus for motor control in an electric vehicle in which a current feedback control for an alternating current (AC) motor is performed by using a digital arithmetic unit.
In conventional motor control equipment, the maximum output frequency of a power converter is in the neighborhood of 400 MHz. When current feedback control for an AC motor is performed at this frequency by using digital arithmetic means such as a microcomputer, etc., the primary current which flows in an AC motor and the rotational phase of the motor are detected first. Then a d-axis current, which controls the magnetic flux of the AC motor by the primary current and the rotational phase, and a q-axis current orthogonal to the d-axis current, are detected.
Feedback control is performed so that the detected d-axis and q-axis currents can match up to the respective commands, and thereby a d-axis voltage command and a q-axis voltage command are obtained. In such feedback control, PI (Proportional Integral) control is generally carried out independently in the d-axis and the q-axis, so that the deviation between the current command values of the d-axis and the q-axis and the detected values can be equal to zero.
An example of motor control equipment in which a current feedback control for an AC motor is performed is disclosed, for example, in Japanese Patent Publication No. 3-1917 (1991). In this example, in consideration of the component of the voltage generated by the inductance of the load in an electric current control system for the AC motor, the deviation between the command value and the measured value of the d-axis current is applied to the q-axis voltage supplied to the power converter, and the deviation between the command value and the measured value of the q-axis current is applied to the d-axis voltage supplied to the power converter. In this manner, it is possible to achieve stable control, without interaction between the d-axis and the q-axis.
Further, an example of the phase correction for the digital control system is disclosed in Japanese Patent Application Laid-open No.6-335277(1994). In this example, the motor control equipment comprises a rotation compensating means for adding the angle of rotation which delays according to the sampling time, another rotation compensating means for adding the angle of rotation which delays according to the sampling hold time, and a further rotation compensating means for adding the angle of rotation which delays according to the data read-in delay time. Thereby, it becomes possible to eliminate the delay of the angle of rotation due to the sampling.
However, the above-mentioned examples have the following problem.
The AC control equipment disclosed in Japanese Patent Publication No. 3-1917 is effective to maintain the stability when the angular frequency of the AC motor increases up to one tenth of the sampling frequency of the arithmetic unit, and thus the sampling error of the phase detection of the AC motor has an effect on the current control system. However, because the angular frequency is directly multiplied by the deviation of the q-axis and that of the d-axis when the deviation of the q-axis current is applied to the d-axis voltage and that of the d-axis current is applied to the q-axis voltage, the gain of the integration may be changed. As a result, there is a concern that the current control system does not have the response as designed. Accordingly, it is impossible to increase the angular frequency of the AC motor, for example, to more than 400 Hz.
In the AC control equipment disclosed in Japanese Patent Application Laid-Open No. 6-335277, the delay of the phase detection is compensated. Therefore, it is effective to maintain the stability when the sampling error of the phase detection of the AC motor has an effect on the current control system. However, when the AC motor is miniaturized and the output frequency of the power converter is increased so that the maximum output frequency of the power converter reaches approximately one tenth of the sampling frequency of the digital arithmetic unit (for example, more than 500 Hz), the impact of the detection error of the rotational phase on the control system cannot be neglected. As a result, in the normal PI control performed independently in the respective axes, the interference component between the d-axis and the q-axis is no longer compensated, and the stability of the control system deteriorates. Further, it becomes impossible to perform current feedback control. Even if it is possible to maintain the stability of the control system, there are concerns that the accuracy of detection of the d-axis current and the q-axis current deteriorates, or the resolution of the speed detector decreases and thus the d-axis current and the q-axis current include a ripple current. Such a ripple current causes pulsation in rotation of the AC motor. Therefore, for example, in the AC motor for an electric vehicle, the running performance and/or the riding comfort may decrease.
Therefore, as in Japanese Patent Publication No. 3-1917, it is also impossible to increase the angular frequency in the motor control equipment disclosed in Japanese Patent Application Laid-Open No. 6-335277.